Do you mean the top right connector assignment box in FZ edit, like above.
Just click on the text writing - like MD1 D1 above - and the line goes dark grey. It's the blank line with no box to indicate that you can click on it.
Then click the button Select Graphic.
Then go to the drawing and click the connector - purple square bottom left -
On the right again under the connector box you can pick Centre, N,S,E,W if you want, by clicking the boxes.
You can click on anything in the list and it will light up in the drawing if it's assigned.